Since the so-called “green problem” was discovered by T. Baer in 1986, it has become well known and has long plagued the stability of the CW intracavity harmonic generation of diode-pumped solid-state (DPSS) lasers. The essential difficulty in solving the “green problem” results from that, there is a persistent obstacle in effectively obtaining single longitudinal mode CW operation due to the spatial hole-burning interference effect in solid-state lasers. The related critical design issues are extremely tough. For the past decade, much research has attempted to solve this problem to obtain stable green light. Almost every effort has been made and nearly every way has been tried. However, none of true CW devices or designs has been successful by far with a regular standing-wave cavity. Only ring or very short cavity configurations have been used for this purpose, but they have appreciable inconveniences and limitations.
Baer, many AMOCO scientists and others did primary works and made some detailed reviews to the “green problem” in their papers and patents, such as the U.S. Pat. No. 5,164,947 (1992) and paper “Intracavity Doubling of CW Diode-pumped Nd:YAG laser with KTP,” IEEE J. QE-28, 1148(1992). Baer recognized the “green problem” and pointed out that there was a fundamental barrier to successful multimode operation of intracavity doubled lasers. (Now, the “multimode operation” should be corrected to be “a few modes operation”.) AMOCO and other scientists examined and worked out several important problems, including minimizing spatial hole burning effect with the “twisted mode” technology and the various polarization related problems, such as modifying the polarization of the laser modes in the doubling crystal to reduce the likelihood of chaotic amplitude fluctuations.
Controlling spatial hole burning can greatly reduce the possibility of amplitude oscillations. However, weak residual spatial hole burning resulted from imperfect “twisted mode” operation can still cause oscillations. In spite of those intense efforts, there remains a determining approach required to achieve dynamically stable single-mode operation with the use of a regular standing-wave cavity when the spatial hole-burning effect is present. What is needed is to provide a powerful form of wavelength selectivity to clamp the peak position of the operating frequency and prevent the laser operation from mode hopping and shifting to wavelengths outside the phase matching curve while controlling appreciable losses to the system.
On the other hand, an etalon within a cavity is commonly used to further control and suppress the harmful mode operation. Etalons typically have the highest spectral mode discrimination. However, the insertion of an etalon often leads to large passive losses and significantly reduces output power. This is especially true, for example, when the etalon is of high-finesse type, or the cavity has a small spatial mode waist and, hence, large beam divergence, and these effects are worse when the etalon is titled. Therefore, as simply inserting an etalon to a laser cavity, these characteristics often lead to the failure of laser operation.
AMOCO scientists realized and considered this key factor and were very close to success. In fact, there was almost one step behind to win the battle of the “green problemn”. Although they did not cross this decisive step, they have demonstrated several important concerns over the unsolved difficulties inherent in the “green problem” under the condition of single-mode operation. Following are the major concerns in their paper.
(1) “The intracavity harmonic generation laser is much more sensitive to component quality and the associated insertion loss than are most other lasers. To build an efficient intracavity harmonic generation laser, one needs to find some forms of mode selectivity with low loss, which is a significantly difficult task. On the other hand, if enough constraints are placed on the cavity without introducing appreciable losses to the system, stable and efficient operation of intracavity harmonic generation lasers is possible.”
(2) “The doubling efficiency is extremely sensitive to the finesse of the laser cavity so that all these controls must be introduced into the laser cavity without adding appreciable loss to the system.”
(3) “The principle difficulty with this design is that combining a polarizer and a highly birefringent element with a relatively small mode radius (w=100 um) can lead to significant losses. It is found that the green and 1064 nm output from cavities containing Brewster plates are often substantially smaller than those in similar cavities without Brewster plates. A 100 um beam has a far-field divergence angle of 3.4 mr; the off-axis components of the beam are appreciably depolarized by the angle-dependent refractive index. The phase shifts are only a fraction of a wave, but in the presence of a polarizer, these correspond to losses on the order of a fraction of a percent.”
In conclusion, their major point focuses on that, a relatively small mode waist can lead to significant insertion losses for the intracavity optical elements, particularly for an inserted etalon or Brewster plate in the present case.